Ectopic thyroid tissue results from abnormal embryologic development and migration of the thyroid gland. True malignant transformation in ectopic thyroid tissue is extremely rare and is always diagnosed after surgical excision of the lesion by pathology examinations. There are well-documented cases of ectopic thyroid cancer while primary tumoral lesion occurs in the orthotopic thyroid, but only rare cases of ectopic PTC without any evidence of occult thyroid cancer in the orthotopic thyroid or cervical lymph nodes have been reported. We report on a 39 year old woman who was operated for a mediastinal mass. The initial diagnosis was a malignant thymic lesion, which was later confirmed to be a papillary thyroid carcinoma. Consequently, total thyroidectomy was performed and pathology report showed normal thyroid tissue with no evidence of any neoplastic involvement. Until now, only one similar case has been reported.